Initially introduced as small-scale imaging devices for business presentation markets, electronic color projectors have steadily improved in overall imaging capability and light output capacity. In order for electronic motion picture projectors to compete with conventional motion picture film projectors such as those used in theaters, however, a number of significant technical hurdles remain. Among these hurdles is the need for providing sufficient illumination for large-scale image projection applications.
The overall light path for electronic color projection can be considered as having three basic subsystems: an illumination subsystem for providing red, green and blue color light at sufficient brightness; a modulation subsystem comprising devices such as spatial light modulators for modulating each color light; and a projection subsystem for combining and projecting the modulated light towards a display screen. A key problem that must be resolved in design of the illumination subsystem of an electronic projector is providing sufficiently bright light for modulation and projection components.
Referring to FIG. 1a, there is shown an illumination apparatus 10 for providing, from a single white light source 20, red, green, and blue light for modulation. Light source 20, typically a xenon lamp or other high intensity white light source, provides light along an optical axis O through uniformizing optics 22, such as an integrating bar or lenslet array and directed by a lens 24 to a color separator prism 30. Color separator prism 30 may have the V-cube or V-prism configuration shown in FIG. 1a, as is disclosed in U.S. Pat. No. 5,944,401 (Murakami et al.) and U.S. Pat. No. 5,357,289 (Konno et al.) Color separator prism 30 may alternately be an X-cube or X-prism, a Philips prism, or similar device. Color separator prism 30 typically uses a set of dichroic coated surfaces on glued-together prism segments, S1, S2, and S3 for separating light into its component colors. Red light for modulation is then directed along red optical axis Or, blue light along blue optical axis Ob, and green light along green optical axis Og.
As can be readily appreciated from the block diagram of FIG. 1a, the role of color separator prism 30 requires handling high intensity light at its input and, therefore, of handling intense heat. Increasing the brightness of individual color channels necessitates increasing the overall brightness of light that must be handled by color separator prism 30. High-temperature glass sections have suitable coatings and are bonded together with optical adhesives. Mechanical drift and stresses from external mounting components can cause slight angular shifting of the dichroic surfaces within color separator prism 30, with negative effects such as color shading and light loss. For this reason, support components for mounting color separator prism 30 within a projector chassis must allow for heat-related factors and must be designed to minimize heat build-up and to minimize the effects of thermal expansion due to unwanted thermal containment of the mounting mechanics.
One conventional solution has been to make color separator prism 30 as large as possible, thereby spreading heat effects over a sizeable mass. However, this coarse approach prevents the design of more compact projection apparatus. Meanwhile, conventional prism mounting techniques for color separator prisms and other heat sensitive prism applications are characterized by mechanical complexity, over-constraint, crowding, and need for precision adjustment and allowance for heat effects. For example:                U.S. Pat. No. 6,181,490 (Wun et al.) discloses an adjustable optical frame used for a prism in an optical combiner application in which a prism is enclosed within a complex sheet metal frame that provides multiple constraints on prism movement and expansion and has numerous adjustments;        U.S. Pat. No. 3,848,973 (Merz et al.) discloses a prism holder for use in a light deflection system, in which a compression assembly is employed;        U.S. Pat. No. 4,571,028 (Ziegler et al.) discloses a prism mount for a prism enclosed in a tube, which would not be well-suited to a high-heat environment due to containment of the prism;        U.S. Pat. No. 5,749,641 (Brice et al.) discloses a color combiner or separator prism enclosed on five sides within a complex frame structure having multiple sections, with some frame sections used to support mounting of other optical components;        U.S. Pat. No. 6,141,150 (Ushiyama et al.) discloses a dichroic prism mounting method using oversized components, requiring complex alignment procedures and presenting demanding adhesive requirements; and        
U.S. Pat. No. 6,010,221 (Maki et al.) discloses a prism mount for a projection apparatus, using a diecast holding member that surrounds the prism in an arrangement that would not be optimal for high heat applications and may over-constrain the prism.
As a rule of thumb, the literature for prism mounting generally recommends using a kinematic configuration, such as mechanical compression, as is discussed in Handbook of Optomechanical Engineering, Anees Ahmad, Editor, CRC Press, New York, N.Y., 1997, pp. 202-210. However, attempts to provide suitable prism mounting using spring forces, frames, or other mechanical constraints have proved inadequate to the task of providing a stable mount for a color separating prism in a high brightness projection apparatus, primarily due to thermal expansion and the need for cooling. Moreover, any type of mechanical device that applies a constraint or preloading force can obstruct the light path, thereby reducing the available brightness. When mounting a V-prism in the illumination path as a color separator, for example, it is necessary to allow, as far as possible, an unobstructed light path at multiple faces of the prism. In general, it is desirable to minimize the size of a color separator prism while, at the same time, maximizing its effective area for light propagation. Because a V-prism or other type of color separator prism is fabricated as an assembly of glued prism components, mounting schemes should minimize, equalize, or eliminate mechanical stress on glued seams. Unwanted stress birefringence can also occur due to constraining forces applied against any prism surface. These same heat and mounting stress considerations that apply for color separator prisms may also apply for color-combining prisms that combine separate monochrome color modulation paths to form a color output beam for projection, and for prisms used to separate or combine light paths in other applications.
Thus it can be seen that there is a need for a prism mounting apparatus and method that is particularly well suited for use with a color separator prism or color combiner prism in a high-energy illumination subsystem of an electronic color projection apparatus.